It's as usual, you get what you pay for.Ok so a DC-DC conerter is unreliable? I mean why would I need a 12v battery? anyways if its recommended I will buy one
A well built DC/DC will of course be extremely reliable, but even a high quality product might fail and when the DC/DC fails your whole car will go black in an instant. If you're driving when it's dark a total blackout will of course be extremely dangerous since you'll lose your head lights.
A DC/DC will also shut down or current limit the output if you temporarely overload it. For example, if you get a short circuit somewhere the DC/DC might cause a black or brown out until the fuse blows or, if you're unlucky, the DC/DC will limit itself to protect the fuse and you'll manually have to figure out where the short is and pull the fuse. Not so good.
A small battery is a good safe guard for many of these problems. A battery doesn't back off like a DC/DC if there's a short circuit but will provide current enough to blow the fuse. Also; a battery will take care of peak currents so you can choose a DC/DC that covers the need for, for example, 90% of the time but that will need a little help from the battery for the initial current rush when you switch something on, like the high beam.
Some people only use a 12 Volt battery instead of a DC/DC, but that has it's own draw backs like that you need a separate charger for it plus that it means your 12 Volt system will be running at ~11.5 Volt rather than ~13.5 Volt (which is the normal charging current for the 12 Volt battery in a car). This will make your lamps yellowish and weak and also mean that the margins for the Soliton 1 will be very narrow (it starts to warn for low voltage at 11 Volt and shuts down at 10 Volt).
In Europe there's also a legal demand that a broken down car must have a battery big enough to provide power to the hazard flashers for at least 24 hours. I don't know if there's similar rules in US as well but it's worth checking out before you start your conversion.